2015 Major happenings:
June campaign announcement Trump Tower
July - Iowa campaign stop - McCain sees as attack (see USAtoday article) https://www.usatoday.com/story/news/nation/2015/07/25/mccain-surprised-trump-attack-says-pows-owed-apology/30670799/
Sept - Trump USA today Op-Ed slamming Obama Iran Deal (amateur hour) https://www.usatoday.com/story/opinion/2015/09/08/donald-trump-amateur-hour-iran-nuclear-deal-column/71884090/
Oct?? (Missing date) signed non-binding letter of intent for Moscow Trump Tower deal.
https://abcnews.go.com/Politics/trump-knew-moscow-tower-proposal-campaign-lawyer/story?id=49472342
One of these is the likely trigger - I'm betting the Letter of Intent.
But it's hard to underestimate McCain and Obama's vengeance.